Pre-press color proofing is a procedure used by the printing industry for creating representative images of printed material without the high cost and time required to actually produce printing plates and set up a high-speed, high-volume, printing press to produce a single example of an intended image. These intended images may require several corrections and may need to be reproduced several times to satisfy customers requirements. By utilizing pre-press color proofing time and money can be saved.
One such commercially available image processing apparatus, disclosed in commonly assigned U.S. Pat. No. 5,268,708, describes image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying a sufficient amount of thermal energy to the dye donor material to form an intended image. This image processing apparatus is comprised of a material supply assembly or carousel; lathe bed scanning subsystem, which includes a lathe bed scanning frame, translation drive, translation stage member, and printhead; vacuum imaging drum; and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is cut into sheets, transported to the vacuum imaging drum, registered, wrapped around, and secured on the vacuum imaging drum. A length of dye donor material, in roll form, is metered out of the material supply assembly or carousel, and cut into sheets. The dye donor material is transported to and wrapped around the vacuum imaging drum, such that it is superposed in the registration with the thermal print media.
After the dye donor material is secured to the periphery of the vacuum imaging drum, the scanning subsystem writes an image on the thermal print media as the thermal print media and the dye donor material is rotated past the printhead. The translation drive traverses the printhead and translation stage member axially along the vacuum imaging drum, in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is removed from the vacuum imaging drum without disturbing the thermal print media that is beneath it. The dye donor material is transported out of the image processing apparatus by the dye donor material exit transport. Additional sheets of dye donor material are sequentially superposed with the thermal print media on the vacuum imaging drum, and imaged onto the thermal print media as described above until the intended image is completed. The completed image on the thermal print media is unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the receiver sheet material exit transport.
The vacuum imaging drum is cylindrical in shape and includes a hollow interior portion. A plurality of holes extending through the drum, apply a vacuum from the interior of the vacuum imaging drum to maintain the thermal print media and dye donor material on the drum as the vacuum imaging drum rotates.
A DC motor stator is attached to the lathe bed scanning frame, encircling a armature to form a reversible, variable speed DC drive motor for the vacuum imaging drum. The opposite spindle is provided with a central vacuum opening, which is in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. Vacuum fitting is connected to a high-volume vacuum blower which is capable of producing 50-60 inches of water (93.5-112.2 mm of mercury) at an air flow volume of 60-70 cfm (28.368-33.096 liters per second). The blower provides vacuum to the vacuum imaging drum to hold the thermal print media and the dye donor materials on the drum while the drum is rotating.
The task of loading and unloading the dye donor materials onto and off the vacuum imaging drum, requires precise positioning of thermal print media and the dye donor materials. The lead edge positioning of dye donor material must be accurately controlled during this process. Existing image processing apparatus designs, such as that disclosed in said commonly assigned U.S. patents, employs a multi-chambered vacuum imaging drum for such lead-edge control. One chamber applies vacuum which holds the lead edge of the dye donor material. Another chamber controls vacuum which holds the trail edge of the thermal print media to the vacuum imaging drum. With this arrangement, loading a sheet of thermal print media and the dye donor material requires that the image processing apparatus feed the lead edge of the thermal print media and dye donor material into position just past the vacuum ports controlled by the respective valved chamber. Then vacuum is applied, gripping the lead edge of the dye donor materials against the vacuum imaging drum surface.
Unloading the dye donor material or the thermal print media requires the removal of vacuum from these same chambers so that an edge of the thermal print media or the dye donor material are freed and project out from the surface of the vacuum imaging drum. The image processing apparatus then positions an articulating skive into the path of the free edge to lift the edge further and to feed the dye donor material, to a waste bin or an output tray.
Although the operation of prior art image processing apparatus is satisfactory, it is not without drawbacks. The donor and receiver media must be held tightly against the surface of the vacuum imaging drum to prevent irregular surface conditions caused by factors such as folds, creases, wrinkles, or trapped air. Such irregular surface conditions could adversely affect the imaging process, or cause the media to fly-off at high drum speeds causing damage to the image processing apparatus. To achieve a flat surface, considerable vacuum force is exerted.
Throughput, the number of intended images per hour, is limited by the vacuum imaging drum rotational speed. The faster the vacuum imaging drum rotates without centrifugal forces or increased air turbulence lifting the thermal print media and the dye donor material from the vacuum imaging drum, the faster the intended image can be printed on the thermal print media. Thus faster rotational speeds will increase the throughput of the image processing apparatus.
Existing image processing apparatus technology is limited by the rotational speeds. At high rotational speeds, speeds in excess of 1000 RPM, centrifugal forces and air turbulence can lift or separate the dye donor materials from the vacuum imaging drum surface if the dye donor material and thermal print media is not correctly positioned on the surface of the vacuum imaging drum. If the dye donor material and thermal print media separates from the vacuum imaging drum, it could cause a media jam within the image processing apparatus, resulting in the loss of the intended image output, or cause catastrophic damage to the image processing apparatus.
Vacuum is applied to the thermal print media and dye donor material by a set of vacuum holes and vacuum grooves in the surface of the vacuum imaging drum, one set of holes and grooves for the thermal print media and one set for the dye donor material. One way to prevent the increased air turbulence and centrifugal force from lifting or separating the dye donor material from the rotating vacuum imaging drum would be to add more vacuum holes or enlarge the diameter of the vacuum holes. This would, however, require an increase in the vacuum level in the interior of the vacuum imaging. While this would allow increased vacuum imaging drum speed, it creates a problem with the loading the thermal print media and dye donor material since the lead edge must slide over the first rows of vacuum holes and grooves without being attached prematurely. Also, removal of the thermal print media and dye donor material from a vacuum imaging drum with more vacuum holes or larger vacuum holes is more difficult.